Power plants using coal-fired combustion chambers are typically supplied with coal from an on-premises coal mill or pulverizer. Ground coal "fines" are delivered from the pulverizer to the combustion chamber, often by means of an exhauster fan or some other source of suction or pressure sufficient to pneumatically carry the dust-like fines from the mill through delivery pipes to the combustion chamber.
Occasionally it is necessary to isolate a section of delivery pipe to allow work on a mill, pulverizer, or exhauster fan. Common sense and certain codes require that the system being worked on be isolated from the combustion chamber to prevent a reverse "puff" of positive pressure (and heat, gas or coal dust) from the combustion chamber in reverse through the pipe.
At the present time, it is believed that the National Fire Protection Agency applies certain standards and requirements to such shutoff valves, for example NFPA Valve Requirements 2-5.1.2.1 (Pulverized Fuel Systems):
"2-5.1.2.1 For a suction furnace that can be fired by other main fuels or is connected to two or more pulverizers or exhausters, valves shall be installed to isolate all burner lines. This requirement can be met with one dust tight and one barrier valve or two dust tight valves. A dust tight valve shall be installed in the burner pipe as close to the furnace as practicable. The second valve shall be installed as close to the pulverizer as practicable. The valves shall be closed prior to entering a pulverizer, exhauster, or fuel piping."
Such valves are not only used for temporarily closing off sections of coal delivery pipe for maintenance, but they may be used to close down certain sections of the piping for as long as several years. They accordingly must be robust, and yet in the case of dust tight valves maintain a tight seal relative to the isolated section of pipe. Other difficulties encountered in the manufacture and installation of such valves include: the release of pressurized fines from the conduit when the valve is opened and closed; the entrapment of fines within various parts of the shutoff valve mechanism; and the need to maintain a dust tight seal around the valve housing or enclosure when the valve is open.